Waste gases from industrial processes often contain small amounts of gas components that render the waste gases unsuitable for discharge to the environment. For example, waste gas streams from semiconductor fabrication processes comprise as major components inert gases, such as nitrogen, argon, but they also contain small amounts of fluorinated hydrocarbons, phosphine, silanes, etc. Similarly, waste gas streams from certain hydrocarbon processing plants mostly comprise gases that can be discharged to the atmosphere, such as carbon dioxide and nitrogen, but they frequently also contain small amounts of hydrocarbons, hydrocarbon derivatives or byproduct gases, such as carbon monoxide, that cannot be discharged to the environment.
Processes for the destruction of the environmentally objectionable gaseous impurities have long been available. Typical of such processes are combustion and catalytic oxidation processes. Such process are not, however, always effective for the destruction of the objectionable gas components. For instance fluorinated hydrocarbons, such as tetrafluoromethane and hexafluoroethane, cannot easily be destroyed by conventional combustion processes.
Even if processes for the destruction of these compounds were available, it is not always desirable to destroy the compounds because of their value. In such cases it is preferable to recover and recycle the trace gases.
Processes for the removal of trace gases from waste gas streams are well known. Among the known techniques is adsorptive separation. It is known to purify gas streams by passing the gas streams through adsorbents which more strongly adsorb the impurity than the other components of the gas stream. Typical of known adsorption processes for purifying gas streams is U.S. Pat. No. 5,417,742, issued to Tamhankar et al, which discloses the removal of perfluorocarbons from gas streams by pressure swing adsorption (PSA) or temperature swing adsorption (TSA) using certain adsorbents which selectively adsorb the fluorocarbons.
Equipment configurations for the adsorption of components from gas streams are generally complex and costly, and usually comprise two or more adsorption vessels and two or more pumps to transfer the various gas streams associated with such processes. Less complex equipment configurations, such as those having one adsorption vessel are known. For example, U.S. Pat. No. 5,370,728, issued to LaSala et al, discloses a single vessel PSA system for separating air into oxygen and nitrogen. A void space gas tank located on the nonadsorbed gas outlet end of the system is used to receive and hold void space gas from the adsorption vessel while the vessel is being evacuated. Upon completion of the evacuation step, the vessel is partially repressurized by returning the void space gas into the vessel via its nonadsorbed gas outlet. This patent also discloses the use of the same pump to both charge feed air into the vessel and to evacuate the vessel. U. S. Pat. Nos. 4,561,865 and 4,948,391, issued to McCombs et al and Noguchi, respectively, disclose single adsorption vessel PSA systems which have a gas reservoir on their nonadsorbed gas outlet ends to hold gas for purging and/or partially repressurizing the adsorption vessel. The Noguchi patent also discloses the use of a gas reservoir to hold feed gas. Although these patents provide simplified one-vessel adsorption systems, these systems would not be suitable for removing strongly adsorbed components from gas streams where it is highly desirable to produce a nonadsorbed gas product that is substantially free of the strongly adsorbed gas, since residual strongly adsorbed gas will remain in the nonadsorbed gas conduits after the void space gas is returned to the adsorption vessel.
Methods and equipment configurations which can be used to remove trace components from waste gas streams so that the waste gas stream can be discharged into the atmosphere, are continually sought. The present invention provides an efficient method and a compact and relatively inexpensive single adsorption vessel system for efficiently removing strongly adsorbed trace gases from gas streams on an intermittent basis.